Generally speaking, a semiconductor device having one or more semiconductor chips assembled on an underlying assembly substrate by solder joints made of a solder material is well known. For example, conventional semiconductor devices such as, transistors, diodes, capacitors, thyristors and other discrete semiconductor chips each having a single function are directly assembled on a substrate by creating solder joints between the devices and copper traces, pads, lines or copper lines holes in the underlying substrate. In addition, for modular type semiconductor chips, such as IC, LSI, System on Chip, etc., assembly of the chips to the substrate can be made by solder contacts between the copper lines, pads etc. on the substrate and metallization pads or bumps on the chip. In addition, heat sink unit and other members may also be joined by solder jointing thereof to the substrate on which the semiconductor chips are assembled.
Pb type and Pb—Sn type solders have long been used as the material for these solder joints. Recently, as a result of a desire to stop using, or limiting the use of, Pb based materials, Sn—Ag type and Sn—Ag—Cu type solder materials have been used to create the solder joints between chips and other devices and the underlying substrate. In particular, when the modular type semiconductor chips are assembled, the Pb type solder material with melting point at about 300° C. is adopted, and, when the heat sink and other members are assembled onto the substrate, a solder material with a relatively low melting point, e.g., 200° C. or lower, is used for their attachment thereto.
As devices such as chips are further miniaturized, and the output power of the electronic devices is increased, the heat generating rate per unit area of the semiconductor devices is increasing. For silicon (Si) semiconductor devices, usually, the operating temperature of the device is around 125° C., and its application temperature is about 200° C. That is, the Si semiconductor devices are used in the environment at temperatures of 300° C. or less. On the other hand, for the high power semiconductor devices made of SiC, GaN, etc., operation of the devices can be carried out with low loss even at a temperature 300° C. or more.
However, when the conventional semiconductor device prepared by assembling semiconductor chips on a substrate by a solder material is to operate in a high temperature environment, the solder material with a low melting point may be re-melted breaking the electrical connection between the semiconductor chips and the assembling substrate. This is undesirable. For semiconductor devices in the prior art, it is hard to work with a high stability in a high temperature environment, that is, the low heat resistance is a problem for them.
As for semiconductor devices with a high heat resistance, the following types of physically and electrically joining of the device to the underlying substrate are available: a semiconductor device prepared by assembling semiconductor chips on an assembling substrate by Au—Sn eutectic solder, and a semiconductor device prepared by assembling semiconductor chips on a substrate using a low temperature sintering method by exploiting Ag nano-grains. However, for the semiconductor devices manufactured using a lead-free high-heat-resistant jointing material, Au, Ag and other noble metals are used in a relatively large quantity, resulting in increased manufacturing cost. This is undesirable.